-48VDC grounding requirement

[ee93]

My question is about grounding requirements for telecom equipment on a cell tower site. There is a rectifier and battery backup unit (24VDC system). A DC/DC converter then converts the 24VDC to -48VDC for the telecom equipment. The telecom equipment is split so that the part of the equipment at the base of the tower has a power distribution unit that supplies -48VDC to the other equipment up in the tower. The DC cable is shielded and the shield is grounded. So the questions are:

- is this scenario not covered by NEC scope (ie interpretation of 90.2 (B)(4))? Does a cell tower site leased by telcos meet this rule? Are telcos considered communications utilities?
- further on 250.162 (A) interpretation ie is grounding required:
-- is this application considered premises wiring?
-- does the nominal -48VDC system fall under the 50-300V scope?
-- would this application meet Exception No. 2 being a rectifier-derived DC system?
- is a separate ground/neutral wire needed in this application running up the tower in addition to the +/- wires?

 

[hillbilly1]

On the ones I have installed, the 24 volt dc powerplant and battery strings are removed, and replaced with a -48 vdc power plant and battery strings that has a -48 vdc to 24 vdc converter. Since it is 48 volts, it does not fall under the 50-300 volt scope. There is no neutral in a -48 vdc system, so a seperate neutral/ground is a mute point. The -48 vdc wiring from the tower terminates to a surge protector, it's been a few months since I did one, so I don't remember if there is a sheild to be terminated in the cable.

 

[ee93]

Thanks for the quick reply.

So the NEC specified scope 50-300VDC is for nominal voltages? Just want to be clear on the interpretation since the word nominal is not present in 250.162, actually it mentions the word operating, which could be interpreted to mean the actual voltage which can be more than 50VDC on a -48VDC system, correct?